SIMD fixes for some compilers

This is just a guess, but it looks like maybe some people's compilers
are having trouble with the implicit conversion of int4 to _m128i. In
several places, I relied on the conversion, so this just changes those
spots to be explicit about it.
Big reorg of fmath.h.

* Some things (like understanding the endianness of the platform) have
  moved to platform.h.

* The stuff particular to supplying things that should be in math.h
  or stdint.h, but aren't on some platforms, has all been moved to
  a new header, missing_math.h.

* Tons of safe_blah (clamped input range) and fast_blah (approximate but
  must faster) have been added, copied from a recent addition to
  Open Shading Language (OSL).

* Moved a whole bunch of stuff around the file to give it some kind of
  sane overall organization.
Use fast_math in aniso texture calculations for speedup.